River Plate beat Lanus 1-0 in the first-leg of their all-Argentine Copa Libertadores semifinal.

Barcelona fell behind early Wednesday and now face a three-goal deficit going into the second leg.

The smart money in this new, year-long version of the Copa Libertadores was always on a final between clubs from Brazil and Argentina -- especially when, purely by coincidence, the draw for the knockout stages put almost all the teams from South America's two traditional powers in separate halves of the draw.

But Barcelona of Ecuador had clearly not read the script in the other half. The club from the port city of Guayaquil got the best out of three Brazilian clubs on their way to the semifinal. A fourth seems very unlikely, though, after Barcelona went down 3-0 at home to Gremio.

The contest was effectively decided in the opening 20 minutes. Barcelona showed their calling card, as quick athletic breaks down the flanks caused the visiting defence some anxious early moments. But then Gremio laid their cards on the table.

In full flow, they have a fine passing game, and they moved the ball well, with Arthur stretching the play with a ball out to left-back Bruno Cortez, who danced to the bye line and pulled a low cross back. Barcelona's Matias Oyola made a hash of his clearance, and the ball ran for the excellent support striker Luan to score, with the aid of a looping deflection off an on-rushing defender.

If Barcelona keeper Maximo Banguera was unfortunate on that occasion, he only had himself to blame for the second goal after 20 minutes. He made the mistake of underestimating Gremio right back Edilson, a free-kick specialist.

The foul was some 30 yards from goal, in the left channel. Banguera lined up a defensive wall that was inadequate and poorly positioned. Edilson did not even have to shoot over it or place extravagant curl on the ball. A routine shot went outside the wall and into the back of the net on the keeper's blind side.

Gremio were two goals up without having needed to work up a sweat -- and they barely made it out of their own half for the next 30 minutes. Barcelona threw everything at them. They are a side that performs best when they have space to launch their attacks, though. Gremio sat back to prevent this very situation. And Barcelona were badly missing their livewire Uruguayan centre forward Jonathan Alvez, sent off in the previous match and suspended for this one.

Luan's brace, which started on eight minutes, has Gremio in good shape to advance.

Nevertheless, they forced chances. Gremio's defensive strategy could have gone wrong, especially straight after the interval, when Barcelona changed both their wide players and attacked with renewed vigour.

Gremio keeper Marcelo Grohe is not at his best on crosses. He does not dominate his penalty area, but he has fine reflexes. They were needed to stop a Damian Diaz backheel from a cross from the left, and then Grohe produced a miraculous, point-blank save, hurling himself to his left to block a close-range volley from centre forward Ariel Nahuelpan after Diaz had flicked on a cross from the right.

At this stage, 2-0 was cruel to Barcelona. But it immediately became 3-0. The third Gremio goal was an example of Luan at his best; linking the play, he encouraged a forward run from Edilson, then timed his run to the edge of the area to meet the cross with a crisp volley that gave Banguera no chance.

It was Gremio's best piece of football of the night. Indeed, it was only in the last 20 minutes that they began to produce the fluent possession play that has been the hallmark of their team over the past few months. But on a night when the result was better than the performance, they would surely seem to have done enough to become the first Brazilian club to make it through to the final of the Libertadores since 2013.

Tim Vickery covers South American football for ESPN FC. Follow him on Twitter @Tim_Vickery.